1. Field of the Invention
This invention relates generally to the field of circuit simulation devices including hardware modelers. More particularly, this invention relates to a method and apparatus for determining the status of device cartridges in a hardware modeler.
2. Background of the Invention
Circuit simulators are in common use for modeling the performance of complex electronic circuits. Often, complex circuits such as microprocessors and custom LSI and ASIC circuits are modeled by one of the actual circuits by using a hardware modeler. Such devices are commercially available as for example the CATS.RTM. hardware modeler which is used in conjunction with the CADAT.RTM. simulation software (both products produced by Racal-Redac, the assignee of the present invention). Hardware modeling has been described in, for example, U.S. Pat. Nos. 4,590,581 and 4,635,218 both to Widdoes, Jr. which are incorporated herein by reference.
Briefly, in hardware modeling, an actual device which is being modeled is used as a substitute for a software description of the device. Software descriptions are more common for simpler devices such as AND gates, OR gates, NOR gates, etc. Test vectors are applied to the device from the simulation software and the response of the device to this stimulation is fed back to the simulation software. In the case of many synchronous dynamic devices such as microprocessors, each time the hardware model is accessed, it must be initialized and each previous stimulation vector must be sequentially applied to the device so that it can be restored to the correct internal state prior to application of the current stimulation vector. Commercially available hardware modelers are arranged as banks or card cages of circuit boards or cartridges carrying the various hardware devices to be modeled. In addition, hardware modelers contain the timing and interfacing circuitry and memory necessary to operate the device to be modeled.
When simulation of a complex system with hundreds or thousands of nodes is taking place, the simulation process can often take hours even with modern high speed computers performing the simulation. This is particularly the case when performing fault simulation on such a circuit. In fault simulation of logic circuits, the circuit is simulated as a good circuit and then repeatedly as a circuit having a fault (short to logic high and logic low) at each node of the circuit. For a network of N nodes, this means a total of as many as 2N+1 simulations are carried out. Fault simulation of complex circuits can take days of computer time to complete.
In, for example the CATS.RTM. modeler, the device cartridges are accessible through a smoked acrylic door to enable the operator to interchange hardware cartridges as required for the simulation at hand or perform other operations requiring access to the cartridge bank. The modeler itself may be connected to a local area network for access as a network resource by many users. The status of each cartridge can be determined by a display light on the cartridge itself. In this product and other existing products, only one cartridge may be active at any given time. However, in the preferred embodiment of present invention, more than one cartridge may be active at any given time.
In order to enhance the execution time of such simulations, it is desirable to operate a hardware model at the highest speed possible. This currently means having clock speeds for master clocks operating in the range of 25 Mhz or higher. As the devices being modeled are capable of operating at higher speeds, even higher clock speeds will be desired so that simulation can proceed more rapidly. This gives rise to problems with electromagnetic radiation emissions from the housing of the hardware modeler. It therefore becomes somewhat difficult to obtain suitable levels of emissions from a housing having a non-electrically conductive (glass, plastic, acrylic, etc.) door. Accordingly, a new mechanism to determine the operational status of a cartridge is required.
Since fault simulations can often occupy days of computer time, it is also desirable to make it more difficult for someone to inadvertently remove a cartridge which is being used by someone else on a network, thereby disrupting or "crashing" a simulation.
The present invention addresses these problems by providing a method and apparatus for display of the status of a hardware simulator cartridge and provides a mechanism to pause execution of the hardware model without disrupting an entire simulation.